1. Field of the Invention
This invention relates to protection of motors and mechanisms driven by motors and particularly to memory disk files where excessive torque is required by the motor to start the disk turning due to seizure of the disk by the transducer after prolonged periods of resting contact of the head (transducer) with the disk surface.
2. Prior Art
Numerous difficulties can result when a motor is coupled to a mechanism that is prevented from turning when power is applied to the motor. These difficulties can include a blown fuse, overheated motor windings, damaged mechanism, etc.
The problem is well known in memory disk technology where a phenomenon known as "stiction" has plagued the operation of disk drives for many years. Stiction is the term applied to the sticking of heads (transducers) to disk surfaces when the disk is at rest. When the disk motor is turned on, stiction resists the turning of the disk until the disk is broken free from the transducer.
Stiction is thought to arise from several mechanisms. Most all surfaces have an afinity for one another but a layer of absorbed gas on two adjacent faces normally prevents them from sticking together. However, if the surfaces conform very closely to one another and are forced into intimate contact over a period of time, the absorbed air is forced out of the interface so that the surfaces will eventually stick to one another. This mechanism for sticking is called the JO-BLOCK effect.
Another mechanism for generating stiction is linking on a molecular level of a lubricant between two mating faces when the faces are stationary with respect to one another.
In the most severe case of stiction, the flexures and gimballs supporting the heads have been damaged on startup. Consequently, anticipation of stiction has dictated that the starting torque capability of motors be normally two or more times greater than required to simply keep the disk turning. In addition, greater strength is normally designed into the transducer support structure. These remedies require increased expense and more space.
In the earlier days of disk file development, the approach to disk file design that avoided the problem of stiction was to load the heads onto the moving disk surface after the disks started turning. (See U.S. Pat. No. 3,855,622 to Truscelli and Provanik.) However, with growing interest in personal computers leading to an effort to reduce the cost of the disk file, the practice was adopted of leaving the head in contact with the disk surface when the file was not in use.
The problem with stiction in memory disks became especially severe with the introduction of lubricated disk surfaces which was done in order to avoid the problem of damage to the head and disk resulting from intermittent contact between the head and disk when the disk got up to speed.
Another factor that has increased the severity of the stiction problem has been that the demand for greater information storage density has required smoother disk and head surfaces thereby increasing the JO-BLOCK effect.
One approach to disrupting the adhesive stiction force was disclosed in U.S. Pat. No. 4,530,021 to Cameron. According to this technique, a short oscillatory force is imposed on the head prior to starting rotation of the disk. This is accomplished by sending a pulsing current through the voice coil to an accutator that is normally used to move the head from track to track. The force due to the micromotion is transmitted through the gimbals that support the head and disrupts the stiction force. However, in using this technique, the required force may be so large that damage to the gimballs may occur. The uncertain magnitude of the force required to disrupt stiction makes it difficult to predict how sturdy the head support structure must be in order to withstand a mechanical shock imposed to overcome the force of stiction.